The Modelling of Flowering Time in the French Apple Cropping Area in Relation to Global Warming

Both an increase of global warming and a trend towards an advance in flowering time in apple trees has been recorded in France since the end of 1980s. This study aimed at improving the modelling of apple flowering time with the purpose of exploring the possible impacts of global warming in the recent past and in the near future. Dates of flowering F1 stage in 'Golden Delicious' apple orchards were collected in three cropping areas in France over the 1976-2002 period which provided a suitable climatic variability for modelling, taking account of the global warming. Original soft-ware, 'Pollenoscope', was used to estimate the parameters related to flower bud chilling and heat requirements, using seven chilling submodels and seven heat submodels. The software allowed an automatic research of optimal combinations of chilling and heat submodels. From more than 1000 tested combinations, 19 appeared optimal for the three areas explaining about 85% of the phenological variability. One of these combinations was used to simulate the time-course changes in endodormancy breaking and F1 stage over the 1976-2002 period. In each area, these simulations highlighted a slight trend towards a delayed end of dormancy and inversely a marked trend towards a shortening of floral primordium, active growth period. These results can account for the observed trend in the advance of flowering time. They suggest the existence of two opposite impacts of global warming in France since the end of 1980s, being a delay in satisfying the chilling requirements and a shortening in satisfying the heat requirements.